1. Field of the Invention
The present invention relates to a belt-type continuously variable transmission for a straddle-type vehicle.
2. Description of Related Art
A straddle-type vehicle equipped with a belt-type continuously variable transmission (CVT) is known in the art (for example, refer to WO2003-085278). The CVT includes a primary sheave to which a driving force from the engine is transmitted and a secondary sheave to which the driving force is transmitted from the primary sheave via a belt. At least one of the primary and secondary sheaves is variable in belt winding diameter, so that a speed change ratio can be controlled by varying the ratio of the belt winding diameter of the primary sheave to that of the secondary sheave.
FIG. 7 is a sectional view of a conventional engine unit 112 described in WO2003-085278. Engine unit 112 includes an engine 113, a belt-type continuously variable transmission (CVT) 114, a reduction mechanism 116, and a generator 129. CVT 114 includes a primary sheave 136, a secondary sheave 137, and a belt 141.
Primary sheave 136 is unrotatably fixed to a crankshaft 120. Primary sheave 136 includes a primary fixed sheave member 136a and a primary moving sheave member 136b. Primary moving sheave member 136b is opposed to primary fixed sheave member 136a. Primary moving sheave member 136b and primary fixed sheave member 136a constitute a substantially V-cross-section belt groove 136c around which belt 141 is wound. Primary moving sheave member 136b can be moved relative to primary fixed sheave member 136a along the axis of crankshaft 120.
A cam plate 143 is disposed on the end of primary moving sheave member 136b opposite to primary fixed sheave member 136a and faces primary moving sheave member 136b. Cam plate 143 is tapered radially outward from primary sheave 136 so as to come close to primary moving sheave member 136b. A plurality of roller weights 144 is provided between primary moving sheave member 136b and cam plate 143. Roller weights 144 are displaced in the direction of the radius of primary sheave 136 and revolve around crankshaft 120 with rotation of primary moving sheave member 136b and cam plate 143.
Secondary sheave 137 is unrotatably mounted to a secondary sheave shaft 138. Like primary sheave 136, secondary sheave 137 includes a secondary fixed sheave member 137a and a secondary moving sheave member 137b. Secondary moving sheave member 137b is opposed to secondary fixed sheave member 137a. Secondary moving sheave member 137b and secondary fixed sheave member 137a constitute a substantially V-cross-section belt groove 137c around which belt 141 is wound. Secondary moving sheave member 137b can be moved relative to secondary fixed sheave member 137a along the axis of secondary sheave shaft 138.
A spring stopper 147 is disposed on the end of secondary moving sheave member 137b opposite to secondary fixed sheave member 137a. Spring stopper 147 is mounted to secondary sheave shaft 138 and cannot be moved relative to secondary fixed sheave member 137a along the axis of secondary sheave shaft 138. A compression coil spring 145 is disposed between spring stopper 147 and secondary moving sheave member 137b. Compression coil spring 145 urges secondary moving sheave member 137b in the direction in which belt groove 137c decreases in width, that is, the direction in which the distance between secondary moving sheave member 137b and secondary fixed sheave member 137a decreases.
When the rotation speed of primary sheave 136 (the rotation speed of engine 113) is low, the width of belt groove 137c is held small by the urging force of compression coil spring 145. Therefore, the winding diameter of belt 141 on secondary sheave 137 is relatively large and belt 141 is drawn to secondary sheave 137. Thus, roller weights 144 are held close to the rotation axis so that the width of belt groove 136c of primary sheave 136 is held relatively large. This results in a high speed change ratio.
When the rotation speed of primary sheave 136 increases, the centrifugal force generated at roller weights 144 also increases. Therefore, the pressure of roller weights 144 onto primary moving sheave member 136b overcomes the urging force of compression coil spring 145, so that primary moving sheave member 136b is moved toward primary fixed sheave member 136a. In addition, compression coil spring 145 is compressed to increase the width of belt groove 137c of secondary sheave 137. Thus, the belt winding diameter of primary sheave 136 increases, while the belt winding diameter of secondary sheave 137 decreases. Accordingly, the speed change ratio decreases as the rotation speed of primary sheave 136, that is, the rotation speed of engine 113, increases.
Conventional CVTs have a problem in that a great amount of vibration is generated, particularly while the engine runs at high rpm.